Photoresist has been used in the photolithography process and the like to form a fine pattern in the semiconductor production processes, for example, for integrated circuits, transistors, and the like. For instance, when a silicon oxide film is formed with a desired pattern on a silicon substrate, the substrate is treated, for example, by the following steps. That is, firstly, a silicon oxide film is formed on the surface of a silicon substrate, a photoresist is coated on the silicon oxide film, after that, a resist film is formed. Subsequently, exposure and development are applied using a photomask corresponding to a desired pattern to obtain the desired pattern. Further, unnecessary oxide film is removed from the substrate having the obtained desired pattern, by an etching process such as plasma doping. Finally, by removing the resist and washing the surface of the substrate, a silicon oxide film forming a desired pattern can be obtained.
As a method for removing a unnecessary resist after the above-described etching process, two kinds of a dry ashing system mainly by ashing such as oxygen plasma ashing (see: for example, Patent Literature 1), and a wet system such as dipping in a stripping solvent containing various additives have been known. The oxygen plasma ashing which is one example of the dry ashing system is a method in which resist is removed by decomposing and ashing by a reaction of oxygen gas plasma and resist. This method is said to be no pollution, but has problems that the substrate surface is susceptible to be damaged unless detection of an endpoint of the reaction is carried out correctly when resist is removed by ashing, and that particles tend to adhere to and remain on the substrate surface. In addition, the method also has such problems that generally an expensive facility is required for generating plasma and the like. On the other hand, as a wet system, a method using, for example, an inorganic stripping solvent has been known, in which resist is removed by converting to an inorganic substance (ashing) by a strong oxidizing power of peroxomonosulfuric acid (Caro's acid) which is obtained by a reaction of hot concentrated sulfuric acid and hydrogen peroxide (see: for example, Patent Literature 2). However, the method has not only a problem that the method has a high risk to use concentrated sulfuric acid at a high temperature, but also another problem that the method cannot be applied to a substrate having a metal wiring such as, for example, aluminum wiring because the hot mixed acid having a strong oxidizing power forms unnecessary oxide on the metal surface or dissolves the metal.
Under such circumstances, a method for removing a resist by a simple and effective method has been desired, which does not cause such problem of a dry ashing system as mentioned above and does not cause such problem that a metal wiring is dissolved shown by an inorganic type stripping solvent represented by the wet system, and an existence of a chemical agent to satisfy this requirement has been desired.